When an engine combusts fuel, particularly diesel fuel, the engine out exhaust includes some particulate matter (PM) (e.g. non-methane hydrocarbons) and some nitrogen oxide gases (NOX). Particulate matter is the result of incomplete combustion of the fuel. NOX is the result of combusting with atmospheric air, which contains a high ratio of nitrogen (N2) to oxygen (O2). Particulate emissions and NOX emissions are regulated in many jurisdictions and must be limited in the tailpipe exhaust. To meet particulate emissions regulations or standards, a particulate filter is commonly used to trap particulate matter from the engine out exhaust, thereby removing it from tailpipe exhaust. To meet NOX emissions regulations or standards, a NOX reduction device converts NOX into N2.
While catching particulate matter, however, the particulate filter can become clogged when high amounts of particulate matter deposit in the particulate filter, which reduces performance of the engine. The process of removing particulate matter from the particulate filter is called regeneration. Regeneration can be performed by exposing the particulate matter to O2 at a high temperature (e.g. 500 to 600 degrees C.) or nitrogen dioxide (NO2) at a relatively lower temperature (200 to 300 degrees C.). Engine out exhaust often contains a sufficient amount of oxygen to regenerate the particulate filter. However, because the temperature required to regenerate with oxygen is higher than a nominal temperature for exhaust during nominal engine operation, using oxygen to regenerate is often referred to as active regeneration. The high temperatures also wear down exhaust aftertreatment components more quickly.
On the other hand, engine out exhaust often contains a relatively small amount of NO2 for regeneration. The small amount of NO2 present results in passive regeneration, which does not require a temperature higher than during nominal engine operation. An oxidation device including an oxidation catalyst in the aftertreatment system can provide increased NO2 amounts by converting nitric oxide (NO) to nitrogen dioxide. However, the increased amount of NO2 is often insufficient to keep the particulate filter from clogging without also using active regeneration. Furthermore, NO2 is an emission that must be limited at tailpipe exhaust. Thus, time periods of active regeneration by O2 are still required to keep the particulate filter from clogging.
There remains a continuing need for improved methods to operate engine systems to meet emissions regulations.